There is a conventionally known electron tube used to detect the presence or absence of a flame on the basis of ultraviolet rays emitted from a flame in a combustion furnace or the like. The electron tube includes a sealed container in which predetermined gas is filled in a sealing manner, two electrode supporting pins that penetrate through both end portions of the sealed container, and two electrodes (a pair of electrodes) that are supported in parallel with each other by the electrode supporting pins within the sealed container.
In the electron tube, when one electrode facing a flame is irradiated with ultraviolet rays in a state where a predetermined voltage is applied across the electrodes through the electrode supporting pins, electrons are emitted from the one electrode due to the photoelectric effect and excited in succession one after another to cause an electron avalanche between the one electrode and the other electrode. Therefore, it is possible to detect the presence or absence of a flame by measuring a change in impedance between the electrodes, a change in voltage between the electrodes, and a change in current flowing between the electrodes. Various methods for detecting the presence or absence of a flame have been suggested.
Of these methods, for example, there is one proposed method in which current flowing between the electrodes is integrated and the presence of a flame is determined when the integrated value is greater than or equal to a predetermined threshold value or the absence of a flame is determined when the integrated value is less than the predetermined threshold value (see, for example, PTL 1).
However, since this method integrates the current flowing between the electrodes, the time for integration is required even in flame extinction. Accordingly, it takes time until flame extinction is detected, consequently making it difficult to detect the presence or absence of a flame rapidly.
In order to solve the problems described above, the flame detecting device disclosed in PTL 2 includes an electron tube in which a pair of electrodes are provided and electrons are emitted between the electrodes when the electrodes are irradiated with ultraviolet rays, an applying portion that applies a periodically changing voltage across the electrodes, a detecting portion that detects a voltage waveform representing a temporal change of the voltage between the electrodes, and an determining portion that determines the presence or absence of a flame on the basis of the voltage waveform detected by the detecting portion.
Since the flame detecting device disclosed in PTL 2 determines the presence or absence of a flame on the basis of the voltage waveform representing the temporal change of the voltage between the electrodes provided in the electron tube, the time for integration or the like is not required and the presence or absence of a flame can be detected rapidly.